Study Finds Multitude of Bacteria on Human Skin

THURSDAY, May 28 (HealthDay News) --Should it worry you that there are 19 different species of bacteria living behind your ears?

Not especially, when you consider that there are 44 species thriving right out in the open on your forearm.

This information comes to you from scientists at the U.S. National Human Genome Research Institute, who have used the same techniques that enabled them to map the human genetic makeup to identify all the bacteria living on human skin.

There are lots of them, of all sorts. In scientific terms, 19 separate phyla and 205 different genera to be found on the 20 sites sampled by the researchers. The diversity of microbial life on human skin was much greater than expected, according to a report in the May 29 issue of Science.

This might strike some people as an odd way to be spending tax dollars, but that's the wrong view to take, said Julia A. Segre, a senior investigator at the institute who took part in the study.

For one thing, there are many potential medical applications of the work, Segre said. "We are using these results already to initiate clinical research," she said. "We want to know what is the contribution of the human microbiota to common and rare skin disorders."

Segre herself is studying the skin disorder eczema. "We are hoping with this project that we can understand the pathogenic processes in these conditions," she said. "And perhaps we can learn how to help healthy bacteria keep pathogens in check."

But there is also the sheer scientific beauty of what is called the Human Microbiome Project, an offspring of the Human Genome Project, which mapped all the genes in the body. The same techniques used to go through the human genetic sequence now can be applied to bacteria, Segre said.

The diversity found as the investigators sequenced bacteria found in moist sites (inside the nose, the armpits, the navel), dry areas (such as the forearm) and oily sites (inside the ear, between the eyebrows, the back of the scalp) was a surprise, she said.

"All our knowledge had been based on what we could culture in the laboratory," Segre said. "Culturing puts a bias on what you can study. You're limited to what you can grow at a certain temperature in enriched media. In culture, you can find what you are looking for, but it's hard to find what is not there."

Even more unexpected was the similarity of the bacteria living in the same sites on different people. "We found that the site was more determining than the individual," Segre said. "In different people's armpits we found the same bacteria, while different parts of the body had very different bacteria."

Diversity is to be expected in bacteria because there are a lot of different species, and now that diversity can be explored in depth, she said. "A revolution in sequencing technology enables us to obtain information of a complexity that is astronomical compared to what was possible just a few years ago," said Segre, who began to work in the Human Genome Project as a graduate student.

"This is fundamental work, defining one aspect of human biology," said Dr. Martin Blaser, chairman of medicine and a microbiologist at New York University, who has done several studies of human skin bacteria. "The human body is a map, and they are putting more sites on the map. This has been a very good advance."

Blaser and his colleagues have published several studies of bacteria that they found on the human forearm. "We began to use molecular methods several years ago, and we continue to do that," he said. "This work is much more comprehensive that ours were, because the tools are more powerful."

Though the new study can be described as basic science, "the identification of complex microbiota is important if one considers many inflammatory diseases of the skin," Blaser said.

His group has just received a National Institutes of Health grant to study the relationship between skin bacteria and psoriasis, Blaser said. "This paper has a lot of fundamental information that is of use to us," he said.